The past week has seen the highest increase of Ebola cases since the outbreak in West Africa began, the World Health Organization said Friday, offering more evidence that the crisis is worsening.

The U.N. health agency warned Thursday that the outbreak in West Africa is accelerating and could eventually infect as many as 20,000 people. So far it has killed more than 1,500 of the 3,000 people it has sickened in Guinea, Liberia, Nigeria and Sierra Leone, according to an official count. The U.N. health agency said Thursday it assumes that in many hard-hit areas, the actual number of cases may be two to four times higher than is currently reported.

In a detailed report on the outbreak Friday, the WHO said more than 500 cases were recorded over the past week, by far the worst toll of any week so far. The vast majority of the cases were in Liberia, but the agency said it was also the highest number of cases in one week for Guinea and Sierra Leone. Nigeria has also recorded a small number of cases.

"There are serious problems with case management and infection prevention and control," the report said. "The situation is worsening in Liberia and Sierra Leone."

Neither of those countries has enough space in treatment centers to handle the tremendous and increasing number of cases, it said.

The region where the three most affected countries meet remains the epicenter of the outbreak, the WHO said. Nearly two-thirds of all cases have been reported in that area. The agency said that the spread of the virus into densely populated cities is causing concern. Monrovia, Liberia's capital, has been particularly hard hit.

The past week has seen the highest increase of Ebola cases since the outbreak in West Africa began, the World Health Organization said Friday, offering more evidence that the crisis is worsening.The U.N. health agency warned Thursday that the outbreak in West Africa is...